Microsporidial infection continues to be a problem for HIV infected individuals and are associated as a cause of persistent diarrhea and systemic disease in these people. Recent studies have also implicated these agents in causing illness to HIV- negative groups like travelers and immuno-competent elderly individuals. The limited studies available with Encephalitozoon cuniculi, a microsporidia that can be easily cultured in the laboratory have demonstrated importance of T cells in protection against the parasite. Amongst the T cell subsets, CD8+ T lymphocytes are primary effector cells responsible for protective immunity with CD4+ and ? T playing an important helper role. Knock out animals lacking either of the two subsets exhibit sub-optimal CD8+ T cell immunity to E.cuniculi infection. Recent studies from our laboratory suggest that IL- 21, a cytokine produced by both CD4 and ? T cells, is important for the induction of multifunctional CD8+T cell response against the pathogen. Neutralization of IL-21 response leads to decreased polyfunctional and increased mono-functional CD8+ T cell response as a result of which they are less protective and the host is unable to clear infection in an efficient manner. Importance of poly or multifunctional CD8+ T cells has recently been associated with increased protection against viral pathogens, although the direct link has yet to established. Thus it appears that IL-21 mediated polyfunctional CD8+ T cell response is key to protection against E.cuniculi infection which poses a risk to HIV infected population. The proposal comprises of three specific aims. In the first specific aim the kinetics o IL-21 response by ? and CD4+ T cell population in response to E.cuniculi infection will be evaluated. The mechanism of IL-21 production and priming of CD8+ T cell response against the pathogen will be assayed. In the second specific aim role of IL-21 in the development of polyfunctional CD8+ T cell effector response will be determined. Further, importance of polyfunctionality in the development of robust long-term response will be analyzed. In the third and final specific aim various strategies which can lead to induction and maintenance of polyfunctional CD8+ T cell response in immunocompromised (like HIV-infected) host will be tested and therapeutic role of IL-21 in this situation will be evaluated. These studies will have fr reaching implications in other opportunistic infections and viral pathogens where development of robust CD8+ T cell response is critical for host protection. PUBLIC HEALTH RELEVANCE: Microsporidial infection poses a problem for immunocompromised patients like those carrying HIV infection. The goal of this proposal is to understand the immune response needed for the controlling this pathogen so that immunotherapeutic agents needed to protect immuno-deficient subjects and be developed